Diesel engines may include a selective catalytic reduction (SCR) catalyst in an emission control system to reduce emissions of nitrogen oxides (NOx) during engine operation. A reductant, such as ammonia in the form of urea or diesel exhaust fluid (DEF), may be injected, e.g., in the engine exhaust upstream of the SCR catalyst, so that ammonia is stored in the SCR catalyst to convert NOx into nitrogen and water.
In some examples, an emission control system may include a diesel particulate filter (DPF) in addition to an SCR catalyst. When a DPF is used, thermal regeneration may be employed to clean the filter by increasing the temperature and burning soot that has collected in the filter. As the temperature of the DPF increases, the temperature of the SCR catalyst may also increase. Ammonia that is used as a reductant in the catalyst may be desorbed from the SCR catalyst when the temperature increases resulting in ammonia slip from the catalyst. The slipped ammonia may exit the tailpipe and enter the atmosphere and/or the ammonia may be oxidized when passing through the DPF to form NOx, thus increasing nitrogen oxide (NOx) emissions.
The inventors herein have recognized that performance of an SCR catalyst may depend on an amount of reductant, e.g., ammonia, stored in the catalyst and that various SCR catalyst conditions may impact its capability in reducing NOx. For example, NOx conversion capability may increase with the amount of ammonia stored in the catalyst, NOx conversion capability may increase with temperature up to some threshold temperature which is typically in the range of operational interest (e.g., approximately 400° C.) and reduce thereafter, and ammonia storage in an SCR catalyst may decrease with temperature.
Thus, the inventors herein have recognized that ammonia storage in SCR catalysts must be carefully managed in order to achieve optimal SCR performance with respect to NOx conversion efficiency. Previous approaches for managing ammonia storage in SCR catalysts use closed loop storage control. Such closed loop approaches may lead to insufficient ammonia storage and reduced NOx conversion efficiencies during certain operating conditions, e.g., following cold starts or thermal events such as DPF filter regeneration events.
For example, vehicle-off (engine off) to vehicle-on (engine on) transitions may cause disturbances in the regulation of ammonia storage in an SCR catalyst for a desired NOx conversion efficiency. For example, during long soak durations (times between vehicle-off, engine-off and a subsequent vehicle-on, engine-on event) with no incoming ammonia, the catalyst temperature may decrease or increase from ambient temperatures and, although an amount of ammonia in the catalyst may have been stored so that the catalyst performs optimally with respect to NOx conversion just prior to the vehicle-off event, ammonia in the catalyst may become under-stored following the vehicle-off event and may therefore perform sub-optimally at a subsequent vehicle-on event. Further, since active ammonia injection may not be possible at cold exhaust gas temperatures following a cold start event (e.g., less than 190° C.), increasing ammonia storage in the SCR catalyst following a cold start event may be delayed so that ammonia storage in the catalyst remains under-stored following the vehicle-on event.
As another example, thermal events, wherein an SCR catalyst experiences active and rapid heating such as during a DPF regeneration event, may lead to substantial depletion of ammonia storage in an SCR catalyst. Thus, following a thermal event, it may be desirable to replenish ammonia storage at rate faster than that provided by closed loop storage control in order to more rapidly regain optimal NOx conversion efficiency following the thermal event.
In order to at least partially address these issues, a method for operating an engine with an SCR catalyst comprises, in response to a vehicle-off event, injecting ammonia during a final exhaust blowdown until a predetermined value of ammonia is stored in the SCR catalyst; and in response to a subsequent vehicle-on event when an amount of ammonia stored in the SCR catalyst is less than the predetermined value, injecting ammonia until the predetermined value of ammonia is stored in the SCR catalyst. Further, in some examples, the method may further comprise, following a thermal event when a temperature of the SCR catalyst is less than an upper temperature threshold, injecting ammonia until a target value of ammonia is stored in the SCR catalyst.
In this way, ammonia storage in an SCR catalyst may be preemptively boosted just after an engine-off, vehicle-off event so as to minimize an anticipated storage deficit at a subsequent vehicle-on event, engine-on event leading to an increased NOx conversion efficiency following the vehicle-on event. Further, ammonia injection for storage in an SCR catalyst may be adjusted to reduce delays in achieving target ammonia storage amounts in the catalyst for optimum NOx conversion efficiency following conditions which deplete ammonia storage in the catalyst, e.g., following cold-starts or thermal events such as DPF regeneration events.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.